The present invention relates to an apparatus and method for the amplification of a laser beam by pumping a homogenous composite source beam through an amplification medium.
Lasers with pulse widths of less than 10 ps offer new processing capabilities in micro-machining industrial applications. However, the processing speeds enabling mass manufacture require pulse repetition frequencies of between 100 kHz to 10 MHz, adjustable to an optimal frequency with average powers in excess of 100 W. Typically, such combinations are not achievable with most laser architectures as the maximum pulse energy is limited by non-linear effects and damage to the laser crystal. Thin disk laser oscillators have achieved powers approaching 150 W, being at the lower end of the power range required, but at fixed pulse repetition frequencies of between 3.50 MHz and 60 MHz, being above the required range, and they cannot easily be adjusted to an optimal frequency for a specific process whilst maintaining the average power.
Currently, master oscillator power amplifiers (MOPA's) are used to obtain the high average powers required. In such apparatus a low power laser master oscillator generates pulses of the required width which are coupled into a power amplifier. The input pulse stimulates emission within the amplifier which is added to the input pulse to create a higher output energy pulse. As both the intensity and fluence are significantly lower than would be achieved within an oscillator having a similar output, the apparatus can achieve higher output power and energies before damage occurs. MOPA's are now being implemented in a number of alternative laser architectures.
In each of these arrangements a single solid-state crystalline medium forms the active region of the amplifier which is typically pumped via one or more laser diode bars. A laser diode bar is a linear array of emitters with a fast-axis, which refers to the vertical axis (perpendicular to the semiconductor wafer) where the beam diverges quickly, and a slow-axis, parallel to the face of the bars. The slow axis (x-axis) and the fast axis (y-axis) are perpendicular to each other and orthogonal to the pump axis (z-axis). Correct energy coupling from the emitters to the solid-state laser medium is crucial if the device is to operate efficiently with stable mode and power, prerequisites of any industrial laser system. The coupling technique must ensure near uniform absorption and heating, both transverse to and along the pump axis. In 4 level laser systems such as Nd: YAG, thermally generated refractive index profiles can lead to beam steering, modal distortion and depolarisation. Whereas some of these effects can be reduced by the choice of crystal geometry, such as a thin slab or planar waveguide, they cannot be eliminated entirely. Additionally, in quasi 3 level systems, such as Yb: YAG, a finite lower laser level population at room temperature can lead to unwanted absorbing regions if the crystal is not uniformly pumped.
EP1318578 to Daniel Kopf describes a pumping system for a regenerative amplifier in which a suitable substantially smooth laser diode pump spot is obtained from a laser diode array source or multiple arrays, by imaging each single emitter of the array or the arrays without focusing into substantially the same spot at the laser medium. Due to the absence of focusing a comparatively low aspect ratio diode laser beam is achieved at the surface of the gain medium with a comparatively low intensity of the spot, which prevents thermal damages and other problems caused by higher intensities in the regenerative amplifier setup.
The pump scheme of EP1318578 is as detailed in FIG. 1. In the fast axis of the laser diode bar A, a fast axis collimator B in the form of a cylindrical lens, with focal length, f1, between 0.2-1 mm, is placed a distance f1 from the laser diode emitters C. This acts to collimate the strongly divergent light in the fast axis. In the slow axis, a cylindrical lens D of focal length f2 is placed a distance ˜f2 from the diode emitters C. This lens acts only in the slow axis. It acts to collimate the beam in the slow axis and creates substantial overlap between the beams created by the individual emitters within the array. Alternatively, by shifting the position of the cylindrical lens slightly the beams can be placed adjacent to each other in the slow axis. Importantly, there is no focussing of the beam in the fast axis.
The purpose of this pump scheme is to create a near circular, or low aspect ratio, beam with a smooth profile at the surface of the gain medium for use in a regenerative amplifier with a thin disk active region as compared to thin slab or planar waveguide structures used in MOPA's. The low aspect ratio and profile of the beam are required to prevent unwanted aperturing and non-uniform heating effects. To create a smooth profile the cylindrical lens in the slow axis does not image the beam but creates a far-field profile, the Fourier transform of the near-field, at a focal length, f2, from the lens. A disadvantage of this arrangement is that the gain in the active region is limited and thus complex regenerative amplifiers are required for MOPA's.
It is known that after the pump beam enters the crystal it is absorbed according to the well known Beers Law, Iout=Iinexp(-az), where Iout is the remaining beam intensity for an input beam intensity of Iin after a propagation distance z through an absorbing medium characterised by an absorption coefficient α. To overcome this exponential decay in beam pump intensity, double pumped arrangements have been proposed. A double pumped arrangement is found in the commercially available INNOSLAB amplifier from, for example, EdgeWave GmbH, Germany and shown in FIG. 2. The pump scheme is within the outlined box.
FIG. 2 shows a twin layout with two planar laser diode bars E, F, each incident upon the laser crystal G through opposing sides with the pump axis being co-linear to the axis of propagation of the amplified or oscillating seed beam. In the fast axis, the beam is imaged directly into the crystal using a telescope with a suitable magnification to generate a pumped stripe height typically between 20% and 50% of the crystal height. In the slow axis, the beam is focussed into a planar waveguide H. The multiple emitter beams are then ‘mixed’ as they travel along the waveguide, resulting in an output with a more homogenous, or uniform, profile. Using a suitable magnification the homogenised beam is then imaged into the crystal so that the width of the crystal is uniformly pumped.
The main disadvantages of this approach are the complexity of the components and the large footprint of the device which is, typically, 500 mm by 500 mm. The footprint is largely determined by the size and complexity of the pump homogenisation optics and waveguides rather than the crystal or pump diodes themselves. Additionally, as the pump axis is co-linear to the axis of propagation of the seed beam, additional optics are required to redirect the pump beam and seed beam relative to each other.